Drill heads for exploration drilling rigs are previously known that are equipped with hydraulically actuated chucks for gripping, holding and rotating pipes constituting the drill string components.
The jaws inside such chucks are radially movable between grip and release positions with the aid of a hydraulically powered actuator.
In order to hold the jaws in place in slots in the chuck spindle and to move them from the grip position into the release position, there are provided urging means effecting the jaw movement away from the grip position axis so as to open the chuck for releasing the grip on the drill string.
In particular, in the known chuck, the back sides of the jaws are tapered and forced against a tapered surface associated with the axially movable actuator.
There have been experienced problems with current urging means in that they sometimes are not capable of fully bringing the jaws from the grip position into the release position thereby preventing a drill string from passing through the chuck.